1. Field of the Invention
The invention relates to a distribution and mixing apparatus and process for use within a two-phase downflow vessel. The invention specifically relates to an apparatus and process for distributing liquid passing downwardly through a mixed phase reactor containing solid catalyst. The invention more specifically relates to an apparatus used as part of a reactant distribution and mixing device used above or between catalyst beds in a hydroprocessing reactor as in a hydrotreating or hydrocracking process.
2. Related Art
U.S. Pat. No. 5,942,162 illustrates a liquid-vapor distribution device for use in downflow reactors. The devices are fitted over holes in a tray, which extends across the internal cross-section of a reactor. The devices force vapor to travel through a vertical slot leading to an upflow tube 3 which delivers the liquid and vapor to the inlet of a downflow tube 1. The downflow tube 1 extends through the holes in the tray.
U.S. Pat. Nos. 3,824,080 and 3,824,081 illustrate an interbed mixing device comprising an assembly for admixing downward flowing liquid phase hydrocarbonaceous compounds with a hydrogen rich gas stream. These references employ several mixing devices placed at an intermediate elevation in the vertical reactor. These devices include a centrally located main mixing device 16. Located below device 16 is a distributor tray 22 having a plurality of distributor caps 24. These caps comprise a cylindrical cap-like member 25 having V-shaped notches to allow the entrance of fluids. Vapor and liquid can enter the cap through the V-shaped notch and then flow through an opening to catalyst beds located below the distributor tray. Distributor caps are evenly distributed across the distribution tray.
U.S. Pat. No. 4,140,625 illustrates a different form of distributor tray comprising a plurality of caps 12 allowing vapor to flow upward into a cap and then downward through a venturi-shaped lower section having a liquid inlet.
U.S. Pat. No. 5,232,283 illustrates a three-layer mixing system used for admixing liquid and vapor in a mid-point of a hydrocracking reactor. This apparatus comprises an intermediate layer comprising a tray having multiple bubble cap assemblies 46. U.S. Pat. No. 5,690,896 illustrates the same sequence of a mixing chamber above a distribution tray having a plurality of bubble cap assemblies across its surface. A further example of this arrangement of a mixer above a distribution tray is provided by U.S. Pat. No. 5,837,208. This reference gives a limited description of the bubble cap assemblies. They comprise a cylindrical inner wall or riser attached to an opening in the distribution tray. A bell-like cap is placed over the cylindrical inner wall but is separated therefrom to define an annular fluid passageway. U.S. Pat. No. 6,183,702 illustrates yet another assembly for installation in the middle of a downflow catalytic reactor. This installation also comprises an upper mixer and a lower distribution tray.
A bubble cap assembly specifically adapted for use in a reactor is provided in U.S. Pat. No. 5,158,714. The assembly comprises a riser covered in part by a cap. The riser is attached over an opening through the deck tray. Like the previous references, the slots or openings in the cap or skirt appear to be uniform in size and shape and uniformly distributed around the base of the cap. This reference describes various mechanical details, such as means to removably attach the bubble cap assembly to the central cylinder, and a dispersion plate located in a lower end of the riser to provide a flow restriction for the two fluid phases to produce a mist which impacts the catalyst below. The cap is supported by extensions 116 attached to the top end of the central cylindrical wall. The slots 137 in the skirt or cylindrical wall 136 of the cap 114 provide a higher liquid level within the annular space defined by the cap 114 and the riser 112 than on the distribution tray 30. The higher liquid level in the annular space is stated to offset any irregularities in liquid level on the distribution tray 30 and insure a substantially uniform gas-liquid flow through each cap assembly, and substantially uniform mixing of gas and liquid.
Others have employed two phase downflow distributors having uniform flow paths on a tray wherein each flow path has the same configuration and is intended to deliver the same flow rate of liquid. The gas flow is also intended to be equally divided among all the distributors. As exemplified above, various designs for the uniform flow paths of the distributors have been suggested. Uniform flow path distributor designs may provide reasonably even distribution of the liquid over the cross sectional area of the vessel below the tray under ideal conditions. Ideal conditions include for example, a level tray with each distributor installed at the same height and operated with the same depth of liquid around each distributor at design vapor and liquid flow rates to the tray. However, when conditions are not ideal, such as is always the case during commercial operations, maldistribution increases. That is, the liquid is less uniformly distributed over the cross sectional area of the vessel below the tray.
Various uniform flow path designs have different operating ranges or rangeability regarding, for example, the range of vapor and/or liquid flow rates over which the particular design is effective. However, when uniformly configured fluid flow path distributors are subject to different liquid levels on the tray, the paths taken by the vapor and liquid through the various distributors are no longer uniform. For example, liquid at a lower level around one distributor must travel a greater distance before entering its downcomer. Under such conditions, each of the uniform flow path distributors does not deliver the same flow rates of vapor and liquid as intended. Therefore, maldistribution of the liquid across the cross sectional area of the vessel below the tray increases.
As described in the previously cited references, two phase downflow distribution devices are frequently used in hydroprocessing, especially hydrocracking and hydrotreating reactors. The following provides a few examples of non-ideal conditions to which such distribution devices are exposed during commercial operations of such reactors.
It is very important to the performance of a reactor that uniform (plug flow) temperature and reactant flow rate profiles are maintained through a catalyst bed. The temperature and flow rate profiles can interact since a maldistribution of either the vapor or the liquid can result in a change in the temperature profile and vice versa. This is highly critical to the successful long term operation of a reactor, such as a hydrocracking reactor in a petroleum refinery where it is desired to run with a single load of catalyst for an extended period of time. Hydrocracking and other hydrogenation reactions, such as hydrodesulfurization, are very exothermic and the performance of the desired reaction thus raises the temperature of the downward flowing reactants. Further, the activity and selectivity of the catalyst is dependent upon the temperature at which it is operated. The exothermic nature of the reaction, therefore, has an effect upon the performance of the catalyst and the overall process. Maldistribution and inadequate mixing can cause localized temperature excursions which lead to differences in catalyst activity and conversion across the cross section of the reactor. This can lead to a reduced selectivity, reduced average conversion or rate of reaction such that it may be necessary to operate the reactor at more severe conditions or to restrict the flow rate of the feed to the reactor in order to meet a desired level of product quality or conversion without exceeding maximum operating temperatures. This will normally reduce overall process selectivity toward desired products, which lowers the economic benefit of the process.
Another detrimental effect of maldistribution of the reactants and localized temperature excursions is a premature decrease in catalyst life caused by increased deactivation of the catalyst. This may be through use of the catalyst at a high temperature or by the deposition of foulants, such as carbon (coke), on the catalyst surface. In any event, it can lead to premature decrease in the average activity of the catalyst in the reactor and a necessity to prematurely shut down the reactor. Poor product quality, lower than optimum operating capacity, and a premature shut down of the reactor all have significant negative economic effects on the operation of a petroleum refinery. The shut down, reloading and restarting of a hydroprocessing reactor is a complicated and time consuming operation which removes both the subject unit and associated upstream or downstream units from operation. This may potentially reduce the throughput of the entire refinery. It is, therefore, an object of the subject invention to provide an improved apparatus for admixing vapor and liquid and distributing liquid in downward flowing reactors employed to perform hydrocarbon conversion reactions.
Several factors in addition to installation variations, such as an out of level liquid collection tray, may lead to uneven liquid levels on the liquid collection tray of a distributor. One of the problems faced in designing reactor internals in a hydroprocessing reactor are the forces generated by the flowing gas and liquid present in the reactors. These forces coupled with high pressures and temperatures tends to result in great stress on the supports and other apparatus within the reactor. Normally, the forces are absorbed by support beams located in the mixing and distribution equipment. However, there may still be some deflection of internal supports and distribution trays. A much more troubling problem occurs when the descending liquid is fed onto the collection tray at only a few points. This establishes liquid height gradients and liquid flows across the tray. The impact of the descending liquid also causes liquid level disturbances and irregularities. The devices employed on the distribution trays, therefore, need to accommodate these conditions. It is a further object of the subject invention to provide a distribution apparatus which is better able to compensate for differences in the liquid depth surrounding the individual distributors located on different parts of a distribution tray.
The vapor and liquid flow rates through a hydroprocessing reactor will often change over time due to changes in the reactor operating temperature or other operating conditions, changes in the composition of the feed, changes in product specifications or the use of a different catalyst. For instance, an increase in conversion in a hydrocracking reactor leads to increased vaporization of hydrocarbons as the products are more volatile. It is therefore a further object to provide a distribution apparatus, which is very tolerant of changes in the flow rate of vapor and/or liquid, especially increases in the vapor flow rate in the reactor.
The operation of a hydroprocessing unit will vary with time due to differences in feed rates, recycle gas rates, and reactor conditions, e.g. temperature increases to counteract catalyst deactivation. Operating variations such as changes in the liquid flow rate to the tray may cause the level of liquid retained on the collection tray to change. It is a further object of the invention to provide a device which is less sensitive to changes in the liquid level on the collection tray.
The invention is a liquid distribution apparatus for use as part within a two-phase downflow vessel, with the apparatus being less sensitive to local differences in liquid elevation or liquid gradients on the collection tray. The apparatus comprises a plurality of individual distribution assemblies mounted on a horizontal flow-obstructing tray. Each distribution assembly comprises an overlying cap or shroud which helps define one or more individual flow channels each having the shape of an inverted U. The fluid flow path begins at passageways in a vertical side wall of the cap providing communication with the flow channel. When two or more flow channels are present in a single distributor assembly, the fluid flow paths may merge in the assembly above the tray. The apparatus has at least two differently configured fluid flow paths such as by having differences in the configuration of passageways to provide different fluid flow rates through the differently configured fluid flow paths of the apparatus. The mass flux of vapor and/or the mass flux of liquid will vary between different assemblies or between the two or more flow paths of a single distribution assembly having multiple flow channels. This improves distribution of the liquid downward to the next level despite irregularities caused by non-uniform construction, liquid level gradients and/or by an unlevel distribution tray. In an embodiment the subject invention features a distinctive structure of the downcomer, which may be circular, rectangular or triangular, and a distinctive structure of the passageways of the overlying cap.
The total structure of any specific fluid flow path sets the resistance to fluid flow, and hence the flow rates of the gas and the liquid through the fluid flow path. Thus, there must be at least two fluid flow paths in the apparatus that differ in configuration such as by size, shape, inlet restriction, outlet restriction, intermediate restrictions, or combinations thereof.